Considering the manufacturing cost and technology, traditional flat panel displays are generally based on glass plates. However, traditional glass plates with a bigger weight are more fragile owing to lower impact resistance and poor flexibility, while plastic substrates can remove these disadvantages thanks to lightweight and better impact resistance. Moreover, a higher degree of flexibility and better processing characteristics of plastic substrate can offer an improved design of new-generation of flat panel display with regard to the profile and warpage, but also underline the future-proof technical development trend.
The plastic substrates commercially available include Polyethylene terephathalate (PET), Ployethersulfone (PBS) and polycarbonate (PC), all of which are not suitable for optical plastic substrates with regard to their optical penetration, glass inversion temperature (Tg), water permeability and surface roughness. Among them, the biggest barrier of applying flexible plastic substrate to Organic Light Emitter Diode (OLED) lies in its extreme sensitiveness to moisture, which has a great influence upon the stability of OLED components. Take PET plastic substrate, for example, the water permeability at 25° is about 14 gm/m2-day, far higher than the tolerance limit. Furthermore, the surface roughness of plastic substrate will affect the image quality of display. In the event of excessive surface roughness, the evaporated organic light emitter layer will likely generate dark spots and point discharge at the surface of backing material, causing damage to the organic light emitter layer and impairing greatly the display quality.
Moreover, traditional plastic substrates are manufactured through Roll-to-Roll Manufacturing and Roll to Sheet Processing modes, both of which require expensive machine tools. Therefore, traditional glass plates and traditional plastic substrates cannot satisfy these requirements of new-generation for lightweight, thin-profile and flexibility. According to existing know-how, vapor deposition process is used to form a deposition film on plastic substrate, but cannot overcome lower optical penetration of traditional plastic substrates. In addition, the plastic substrates for packaging purpose are not applied for optical display. According to U.S. Pat. No. 5,939,189, the flexible plastic substrates are fitted with an antireflection coating to reduce color reflection. Yet, this example only represented traditional PET plastic substrates with an antireflection coating, which has not realized higher optical penetration, lower water permeability and surface roughness.
As flexible displays are universally available, the flexible displays made of traditional plastic substrates will not only encounter difficulty in manufacturing process, but also affect the product quality and performance to a great extent. Despite of lightweight, good impact resistance and better flexibility, traditional plastic substrates have a poor water permeability and surface roughness. Thus, there is an urgent demand to bring you a key technology of improving plastic substrate's water permeability and surface roughness.